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4hv.org :: Forums :: Projects
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My 4Mhz amplitude modulated audio sstc

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Reaching
Mon May 05 2008, 05:45PM Print View
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
a while ago i thought that i had to start something new. drsstc design is great but after 12 drsstcs i said "hey, what am i doing here.?" so this was the day i started reading. richies site and his 4mhz sstc project made me thoughtfully.
but i wanted something new, something cooler. so i started reading about amplitude modulating. well, here it is.

In detail its just a basic Class e amplifier like richie burnett used. i simplified his shematic and played around and this is what i came up with.





the 4 mhz class e part works great and at the moment i still think about the idea to get rid of the big transformer for the audiosignal. unless it sounds really great.



this was the first test of the prototype, so its all really basic.
what i want to try is to use a linear regulator like everyone here knows, for powersupplys etc. the linear regulator is set to around 90% of the input voltage. with a smallsignal amplifier the audiosignal is amplified and then fed to the gate of the linear reg causing the output voltage to change. like this.





in theory i should have a output voltage which isrelatively stable over a wide current range with a modulated audiosignal on top of the dc voltage. in my simulations it all works really great and with the amplification factor and output voltage carefully set, the power dissipation should be less than 30Watts for 120Watss output power. my first tests showed me that the class e part works best with an input voltage of around 50 volts,.

im on holiday now, so this project will be updated as soon as i tried this linear reg. for sure, the shematics shown here are just for showing what i mean. the final circuits are much more complex.
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Mon May 05 2008, 11:01PM
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2435
Sounds like a great project, you should be able to get some really nice sounding audio out of it with such a nice high Fo. It is hard to judge audio quility from a youtube video, but it sounds like you have some kind of nonlinearity in your system (saturating the audio transformer?) that is causing some distorsion.

I hope you can get the liner reg to work!
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Steve Conner
Tue May 06 2008, 10:50AM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You'll need some kind of RF bypass capacitor between the output of the modulator and the primary coil of the Class-E stage. It's hard to see how it could have worked well without one.
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Reaching
Tue May 06 2008, 11:20AM
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
mh, maybe i was lucky with the settings. this is my first class e project, so what kind of rf bypass capacitor do you mean? a cap in parallel with the primary, tuned at resonant frequency? ok, the mosfet got warm in my circuit but i ran it for some hours and nothing bad happend. input voltage is around 40-50volts but i tested 100volts as well. sure, the design is not perfect, but for now it works fine.
maybe someone who already build class e stages can give me some hints to improve my circuit.

well, there was some disortion due to the audio transformer in my previous circuit. the dc current flowing through the transformer caused the transformer to saturate very quickly, and maybe the bad quality of the selfmade mosfet amp caused more disortion.. thats why i try to get rid of it with this linear reg design. today i want to build it and do some research and tests with different loads to see how rugged it is and how much power dissipation i have to expect.

theres still a lot to do, and thats why i thought to open a project thread to get some suggestions from you
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Tue May 06 2008, 06:48PM
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2435
I think that he means you need to add a cap from the v+ side of L3 to ground, because as it is the inductance/resistance/losses in general of the audio transformer is being added to L3.

The size for that cap is tricky because having too large of one will kill the high frequency performance.


Also, something that would be interesting to see would be to run this coil strait off of a 'real' audio amplifier. If you coil is drawing 100w at 50v that is 25omhs of impedance--which should be able to work with an audio amplifier without too much trouble. I suppose the biggest problem would be dealing with the nonlinearities of the coil, although it should be possible to inject a DC bais after the dc blocking cap in the amp, although by the time all of that is working it would probably have been easier to just build your own modulator they way you were planning....
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uzzors2k
Tue May 06 2008, 09:07PM
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Asker, Norway
Posts: 1308
Awesome, I'm looking forward to seeing what you come up with!

Sorry, but no constructive criticism.
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Marko
Wed May 07 2008, 10:17AM
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Richie Burnett got around all this a much simpler way as I recall -

He just used two class E amplifiers in push pull, and phase modulated one amplifier with audio.

I don't know how he done that, possibly with some sort of crazy PLL circuit.
Power levels and frequencies were of course immense (18Mhz and >1000W with just two IFP460's, I think. Hope I'm not letting out any of his secrets here ).

I don't know much more than that but apparently it is not really worth trying to use some kind of huge amplifier for audio modulation.

I'm not sure how this phase shift modulation sounds, but it may be suffering from supply voltage sag since there's no feedback of any kind to compensate it.

I hope Richie can explain this better some time,

Marko

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Reaching
Wed May 07 2008, 01:23PM
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
well. ive build the linear reg and was surprised.
its really difficult to adjust, but it works quite well. its obviously not that powerful than the audio transformer design, mostly due to the induced extra voltage in the transformer design.

40volts is fine, but the modulation ratio is really small and so on the volume is not as good as i expected.

now, that i know, that this design is worth a try, i want to go further. a new linear reg design for voltages up to 100v is the goal.

after that, the prototyping is almost done and i can start working on my stereo design
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GeordieBoy
Wed May 07 2008, 02:54PM
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 873
If you want to go down the linear high-level AM route I would recommend investigating the Class-H modulator. This arrangement uses a split rail supply and two linear pass elements to give a very respectible efficiency and low distortion for typical audio signals. The modulation efficiency at the quiescent operating point can be nearly 100% because one pass element is in saturation and the other is in cutoff. Excursions of the audio signal either take one device closer to cutoff, or the other device further towards saturation depending on the polarity of the audio waveform. This situation is a significant improvement on a Class A high-level modulator, but without the complexity of a full-blown PWM switching amplifier.

Some info is here:

(It also doesn't require the special modulation transformer to resist saturation due to the DC flowing in the secondary.)

The dog's bollox of high-level AM modulation is the polyphase Class D switching amplifier, but things like this are difficult for a hobbyist to design, build and set up. Or as Marko eluded there are alternative low-level means of introducing modulation such as FM and out-phasing.

-Richie,
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Reaching
Wed May 07 2008, 05:23PM
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
hey thanks. maybe i´ll try it.

finished the linear regulator prototype and build everything on a larger heatsink.

made a new video with a better cam so you can imagine how well this thing sounds. :)

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Firefox
Wed May 07 2008, 08:36PM
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
Wow, that is awesome! Could you add some values to the schematic? This looks exceedingly simple, and I would like to build one like it to show off in physics class.
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Reaching
Wed May 07 2008, 09:13PM
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
yeah, its pretty simple. next week or so i can get the shematics finished. the shematics shown above do not represent the whole circuit. its a bit more complex than that but compared to other circuits its really simple.
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Reaching
Fri May 09 2008, 10:30AM
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
New Video



I build a new secondary and tuned the class e circuit.i nearly doubled the power and now i think its fine :)
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Dago
Fri May 09 2008, 11:23AM
Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
Reaching wrote ...

New Video



I build a new secondary and tuned the class e circuit.i nearly doubled the power and now i think its fine :)


Looks and sounds great, looking forward for more information about it. Does the interference (the sizzle or something that appears in the chorus of the first song) come from the coil or does the camera/microphone just get interference from the coil?
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Reaching
Fri May 09 2008, 11:44AM
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
this noise that appears from time to time is due to the disortion of the microphone. sorry, but i have no better cam for recording. in reality it sounds like some of this ultra high fidelity speakers.
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Dr. H.
Fri May 09 2008, 12:43PM
Registered Member #931
Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 485
Awesome Reaching. It will be very cool when you box it

Cheers Huben
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WaveRider
Fri May 09 2008, 01:55PM
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 496
Very nice! The transformer modulation scheme in the drain circuit is a good old-fashioned AM transmitter method for very linear modulation. The drawback is that the audio amplifier must be powerful enough to provide the depth of modulation you need...up to 50% of the total AM signal power for 100% modulation as well as a big enough transformer to avoid saturation (altho' air-gapping the core reduces this effect)...

If you are going to the trouble to design a regulated DC powersupply, why not design a system where you apply the modulation to the regulator feedback. In effect, using the regulator as an audio amplifier that also handily provides DC bias? Then you would only need a small audio signal...and the feedback would help linearity too!

Also, with deep modulation, you get power-dependent detuning of the tesla coil as the discharge size/temperature is modulated. This introduces non-linearities in the acoustic generation. For this reason, a tracking oscillator using a power oscillator or PLL circuit are recommended.. (in fact, most plasma tweeters I have seen use vacuum-tube power oscillators that are "self-tuning"..)

Great work!!!!!



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GeordieBoy
Fri May 09 2008, 03:46PM
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 873
If you are going to the trouble to design a regulated DC powersupply, why not design a system where you apply the modulation to the regulator feedback. In effect, using the regulator as an audio amplifier that also handily provides DC bias?


Yeah, that is basically what the Class H modulator does. Except it makes use of a split rail supply so that dissipation in the modulator is minimised around the quiescent operating point (normal carrier level.)

By the way, has anyone investigated using Carrier Level Control (DCC) on an audio-modulated Tesla Coil? It would seem like a good way to reduce power draw, minimise corona hiss during quiet passages, and maximise the peak audio volume achieveable with a given set of power semis.

-Richie,
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Reaching
Sun Jun 15 2008, 10:13AM
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
ive been working on this project for weeks now and made a board, which includes the whole circuit.

its fully analog, without an ic or something. there is a crystal oscillator with 4 mhz for the drive signal. amplified by a simple bc546 transistor and a push pull stage of bd139/140, then fed to a gdt to the class e stage.

the audiosignal is amplified by another simple bc546 with emitter current feedback to provide simple adjustment. a dc bias pot with a smal cap in parallel is for adjusting the dc bias of the linear regulator. you adjust this pot for around 30 to 35v output. the amplifier stage is adjusted to provide around 20-30% modulation of the mosfet gate.

both mosfets are cheap irf630, and all the other parts are easily availiable too. all you need is to plug it in and enjoy :) a transformer with 30V AC and 3-4A current is recommned. the ac voltage is rectified on board and smoothed with 13200µF 50V of capacity.



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Myke
Sun Jun 15 2008, 05:34PM
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
Looks very cool. Nice work.
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uzzors2k
Sun Jun 15 2008, 06:56PM
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Asker, Norway
Posts: 1308
Wonderful. Is the sound quality really crystal clear, and is there much/any noise from the streamers? My PLL class E coil is useless for audio modulating because the streamers hiss so loudly. I'm not sure if it's the PLL, my PSU or the low frequency that does it.
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Reaching
Sun Jun 15 2008, 07:06PM
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
the streamers are almost silent, and around 2cm long. there is no hiss from the flame itself
sound is wonderful, crystalclear, yes. :)
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Firefox
Mon Jun 16 2008, 02:02AM
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
That is a beautiful thing *starts scrounging for parts*.
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Arcstarter
Mon Jun 16 2008, 02:49AM
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2252
Firefox wrote ...

That is a beautiful thing *starts scrounging for parts*.

Haha i do the same thing every time i see someone else's projects.
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teslacoolguy
Mon Jun 16 2008, 03:56AM
Registered Member #1107
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Posts: 792
Can you post the pcb files for this thing?
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GeordieBoy
Mon Jun 16 2008, 10:01AM
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 873
Reaching - What material are the two toroids? I assume the RF drive transformer is Mn:Zn ferrite, and it looks like your RF choke is using the same material? A closed ferrite toroid is not really the best choice for the PA's drain choke, because it can saturate due to the DC current component. You might see better performance (better efficiency and cooler PA MOSFET) using a Type-2 iron powder toroid, or a gapped ferrite or stick inductor here. These generally work better as RFCs than ferrite with no air-gap.

Just a comment. Cool project and a nice PCB too!

-Richie,
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Marko
Mon Jun 16 2008, 12:48PM
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys,

Yes, for drain chokes, best to use are the yellow-white iron powder toroids from SMPS output chokes. The choke only sees DC so there is no need to be worried about dissipation on it.

One thing I was personally wondering about from time to time - what *assures* that volt-seconds on the link choke remain constant in class E amplifier? While device is the choke sees full bus voltage for full time, but during off-time wouldn't it see sinusoidal voltage slope?
Why doesn't this disbalance the Vs on the choke?

Marko
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GeordieBoy
Mon Jun 16 2008, 02:24PM
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
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Posts: 873
No don't use yellow/white iron powder (type-26) for Radio Frequency chokes (or green/blue type-52 for that matter! These materials are very lossy above a few hundred kHz and will turn a nice crispy brown colour quite quickly when exposed to rapid large flux excursions. You ideally want carbonyl iron powder like the red/clear type-2 toroids commonly used by radio hams. I forget the other type, maybe 6 or 8? Gapped ferrite is okay though.

The drain choke sees a voltage equal to the supply voltage minus the drain voltage of the MOSFET. The time-averaged value of the applied voltage is zero. Energy that is pumped into the magnetic field when the switch is closed, is released to the resonant tank circuit when the switch opens. It is similar to a boost or flyback converter in that respect.

-Richie,
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Marko
Mon Jun 16 2008, 02:30PM
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
No don't use yellow/white iron powder (type-26) for Radio Frequency chokes (or green/blue type-52 for that matter! These materials are very lossy above a few hundred kHz and will turn a nice crispy brown colour quite quickly when exposed to rapid large flux excursions. You ideally want carbonyl iron powder like the red/clear type-2 toroids commonly used by radio hams. I forget the other type, maybe 6 or 8? Gapped ferrite is okay though.


I don't see what is the problem, how you mean this? Since DC link choke carries only DC current with very small ripple, although it sees AC voltage. I always use them on royer oscillators without any problems.

Matt Bingham has used them a lot for class E amplifiers at very high frequencies without any problems too.

I thought you used them as well Richie, for class E amps. Why aren't all our chokes frying then?

Long ago I was also afraid of using the powder toroids for link chokes thinking they are 'lossy' but everyone assured me against that.
And indeed they work rather well, once enough inductance is used to keep ripple small... (??)
High frequency actually seemed to be beneficial by allowing to use less inductance.
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GeordieBoy
Mon Jun 16 2008, 03:09PM
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It depends on the power level you are running at Marko. Type-26 is the cheapest crapiest grade of iron-powder for use in very high-volume low-cost SMPSUs operating the tens or low hundreds of kHz. If it suffices for any other particular application then great, because it is dirt cheap, but there are many better materials for RFCs.

My point was that at high power levels and high frequencies where only the right materials will survive, type-2 Fe powder is best for minimal heating as the drain choke.
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